Molecular imaging by magnetic resonance requires a smart molecular probe containing a contrast agent that is capable of selective tissue-targeting to label a specific molecular entity within the tissue of interest which is detectable by magnetic resonance imaging (MRI).
One particularly promising application for molecular imaging by magnetic resonance is the diagnosis and detection of Alzheimer's Disease (AD). One of the pathological hallmarks of AD is the extracellular accumulation of amyloid-β (Aβ) peptide into plaques. These plaques are essential for the definitive diagnosis of AD, which is usually confirmed only at postmortem. At present, there is no method for direct imaging of individual β-amyloid plaques in humans that would provide a definitive premortem diagnosis of this disease or a method of measuring disease progression. MRI has a spatial resolution of 30–50 μm, which at least theoretically, has the capacity to resolve individual plaques (neuritic plaque size in an AD patient varies from 2–200 μm).
Radioiodinated human Aβ40 has been used as a molecular probe which binds to β-amyloid plaques both in vitro and in vivo (Wengenack, T. M., et al., Nat. Biotechnol. 18:868–872, 2000). This in vivo binding of plaques was demonstrated with radioiodinated, polyamine-modified, human Aβ40 following intravenous injection in a transgenic mouse model of AD. Furthermore, by covalently attaching gadolinium-DTPA to polyamine-modified Aβ, we have been able to selectively enhance individual plaques by MRI performed on the ex vivo AD mouse brain at 7 T with a spatial resolution approximating plaque size (62.5 μm3) (Poduslo, J. F., et al., Neurobiol. Dis. 11:315–329, 2002.
The ability to quantify the permeability of peptides and proteins at the blood-brain barrier (BBB) (Poduslo, J. F., et al., Proc. Natl. Acad. Sci. USA 9:5705–5709, 1994) has allowed the evaluation of different protein modifications that might be used to enhance this permeability (Poduslo, J. F. and Curran, G. L., Proc. Natl. Acad. Sci. USA 89:2218–2222, 1992; Poduslo, J. F. and Curran, G. L., Molec. Brain Res. 23:157–162, 1994; Poduslo, J. F. and Curran, G. L., J. Neurochem. 66:1599–1609, 1996). In particular, covalent modification with the naturally occurring polyamines, such as putrescine, spermidine, or spermine, has resulted in dramatic increases in the BBB permeability of a number of proteins (Poduslo, J. F. and Curran, G. L., supra, 1996; Poduslo, J. F. and Curran, G. L., J. Neurochem. 67:734–741, 1996; Wenganack, T. M., et al., Brain Res. 767:128–135, 1997; Poduslo, J. F., et al., J. Neurochem. 71:1651–1660, 1998; Poduslo, J. F., et al., J. Neurobiol. 39:371–382, 1999; Poduslo, J. F., Ann. Neurobiol. 48:943–947, 2000). Indeed, polyamine modification of human Aβ40 as described above not only resulted in a significant increase in the BBB permeability, but it also resulted in enhanced binding to amyloid plaques in AD brain sections (Wengenack, T. M., et al., supra, 2000; Poduslo, J. F., et al., supra, 2002).